"Quinone Millipedes" Reconsidered: Evidence for a Mosaic-Like Taxonomic Distribution of Phenol-Based Secretions across the Julidae

Scenting Ketones in the Defense Glands of Two Julids From the Caucasus (Arthropoda, Myriapoda, Diplopoda, Julida)

Millipedes have developed specialized chemical defenses against predators and parasites. Here we investigated the strong repellent odour emitted by two julids from the Caucasus, Syrioiulus continentalis (Attems, 1903) and Pachyiulus krivolutskyi Golovatch, 1977. By NMR and GC-MS, as well as compound synthesis, we identified the major compound in the secretions of both species as 4-ethylhex-1-en-3-one. A second compound was tentatively identified as the dimer of the main compound as 2-ethyl-1-(6-(pentan-3-yl)-3,4-dihydro-2H-pyran-2-yl)butan-1-one. Both compounds amounting for more than 95% of the whole secretion. 4-Ethylhex-1-en-3-one was found for the first time as a naturally occurring compound, and due to its identification in millipedes of the order Julida we propose here the trivial name "julidone". Considering current knowledge, the replacement of the juliform quinones by novel components such as ketones appears to be highly derived conditions in the chemistry of Julida, possibly triggered by different environmental challenges arising from the epigeic life-style of the two species.

The millipede tribe Brachyiulini in the Caucasus (Diplopoda, Julida, Julidae)

The diplopod tribe Brachyiulini is represented in the fauna of the Caucasus by eight genera and 32 species, of which one genus and 14 species are described as new: Colchiobrachyiulusmontanus Vagalinski, sp. nov., Iraniulustricornis Vagalinski, sp. nov., Omobrachyiulusarmatus Vagalinski, sp. nov., O.fasciatus Vagalinski, sp. nov., O.faxifer Vagalinski, sp. nov., O.kvavadzei Vagalinski, sp. nov., O.lazanyiae Vagalinski, sp. nov., O.ponticus Vagalinski, sp. nov., O.pristis Vagalinski, sp. nov., O.trochiloides Vagalinski, sp. nov., O.unugulis Vagalinski, sp. nov., O.zuevi Vagalinski, sp. nov., Svaniulusryvkini Vagalinski, gen. nov., sp. nov., and S.waltheri Vagalinski, gen. nov., sp. nov.Colchiobrachyiulus Lohmander, 1936, a former subgenus of Megaphyllum, is here elevated to a full genus, and the genus Grusiniulus Lohmander, 1936 is downgraded to a subgenus of the genus Cyphobrachyiulus Verhoeff, 1900, both stat. nov., with their previously described species, Colchiobrachyiulusdioscoriadis (Lignau, 1915) and Cyphobrachyiulusredikorzevi (Lohmander, 1936), respectively, listed as comb. nov.Omobrachyiulusbrachyurus (Attems, 1899) is formally established as a junior subjective synonym of O.caucasicus (Karsch, 1881), syn. nov., and Omobrachyiulusimplicitusritsensis (Golovatch, 1981) is formally synonymised with the typical Omobrachyiulusimplicitus (Lohmander, 1936), syn. nov.Omobrachyiulussevangensis (Lohmander, 1932), originally described in the genus Megaphyllum, is here transferred to the former genus, comb. nov. The diagnoses and descriptions of some genera and subgenera are refined and complemented. A key is given to all genera and species of Brachyiulini that occur in the Caucasus, and their distributions are mapped. Several species are recorded as new to the faunas of Armenia, Azerbaijan, Georgia, or Russia. The distribution patterns of the Caucasian Brachyiulini and their biogeographic implications are discussed.

Initial defensive secretory compounds emitted from the live millipede and the induction of apoptotic cell death

The initial defensive secretory compounds emitted from a live millipede have not yet been clarified. This study focused on elucidating the initial secretory compounds emitted from a live millipede. Pre-concentration of the defensive secretory volatile organic compounds (VOC) from the live Polidesmida millipedes, Chamberlinius hualienensis and Oxidus gracilis, was performed using a three-stage VOC concentration technique by an on-line GC/MS system. As a result, the monoterpenes derived from the plant metabolite; i.e., α-pinene, α-thujene, β-pinene, 3-carene, β-myrcene, β-phellandrene, γ-terpinene, o,m,p-cymenes, limonene and camphene were first detected as the initial secretory substances. It was elucidated that some plant monoterpenes have a repellent effect and antifungal and antibacterial actions which are used as defensive substances. In addition, this study also confirmed that these monoterpenes induced apoptotic cell death involved in the induction of the caspase 3/7 activity. The millipede feeds on fallen or withered leaves containing the monoterpenes. Thus, the millipede accumulates the plant defensive secretions in the exocrine defense glands of the body somites, which would be used as against predators.

Wild Blonde Capuchins (Sapajus flavius) Perform Anointing Behaviour Using Toxic Secretions of a Millipede (Spirobolida: Rhinocricidae)

Defensive secretions of millipedes are remarkable for containing toxic quinones known to efficiently repell hematophagous arthropods. Here we show that Endangered blonde capuchin monkeys make use of such secretions. We (i) describe the anointing behavior performed by the monkeys (ii) identify the millipede species used in the process (iii) describe the volatile chemical composition of its secretion. The blonde capuchin monkeys selectively searched for millipedes hidden under the ground. We observed three bouts of anointing behavior, performed by 13 individuals of all age classes (from adults to independent infants), both solitarily (1 event) and socially (10 events). The only millipede species used by the monkeys is an undescribed species of the genus Poecilocricus (Spirobolida, Rhinocricidae). The volatile chemical composition of the secretions was predominantly comprised of a mixture of benzoquinones and hydroquinones. The social nature of the behavior and time of the observations (mosquito season), suggest that social bonding and mosquito avoidance is linked to the anointing behavior of the monkeys.

Biosynthetic origin of benzoquinones in the explosive discharge of the bombardier beetle Brachinus elongatulus

Bombardier beetles are well-known for their remarkable defensive mechanism. Their defensive apparatus consists of two compartments known as the reservoir and the reaction chamber. When challenged, muscles surrounding the reservoir contract sending chemical precursors into the reaction chamber where they mix with enzymes resulting in an explosive discharge of a hot noxious chemical spray containing two major quinones: 1,4-benzoquinone and 2-methyl-1,4-benzoquinone (toluquinone). Previously, it has been speculated that the biosynthesis of all benzoquinones originates from one core precursor, 1,4-hydroquinone. Careful ligation of the base of the reservoir chamber enabled us to prevent the explosive reaction and sample untransformed reservoir fluid, which showed that it accumulates significant quantities of 1,4-hydroquinone and 2-methyl-1,4-hydroquinone. We investigated the biosynthetic mechanisms leading to quinone formation by injecting or feeding Brachinus elongatulus beetles with stable-isotope-labeled precursors. Chemical analysis of defensive secretion samples obtained from 1,4-hydroquinone-d₆-administered beetles demonstrated that it underwent conversion specifically to 1,4-benzoquinone. Analogously, results from m-cresol-d₈ injected or fed beetles confirmed that m-cresol is metabolized to 2-methyl-1,4-hydroquinone, which is then oxidized to 2-methyl-1,4-benzoquinone in the hot spray. Our results refute the previous claim that 1,4-hydroquinone is the precursor of all substituted benzoquinones in bombardier beetles and reveal that they are biosynthetic products of two independent pathways. Most likely, the aforementioned biosynthetic channel of hydroxylation of appropriate phenolic precursors and subsequent oxidation is not restricted to bombardier beetles; it could well be a general pathway that leads to the formation of all congeners of benzoquinones, one of the most widely distributed groups of defensive compounds in arthropods. Graphical abstract.

Phenol-based millipede defence: antimicrobial activity of secretions from the Balkan endemic millipede Apfelbeckia insculpta (L. Koch, 1867) (Diplopoda: Callipodida)

Millipedes use an array of chemical compounds to defend themselves from predator attack. These chemical substances can have additional roles, i.e. defence against various pathogens. We evaluated the efficacy of the defensive secretion of Apfelbeckia insculpta (L. Koch, 1867) against bacteria, yeasts, and filamentous fungi. The tested secretion consisted of two compounds, p-cresol and phenol, and showed antibacterial, antibiofilm, and antifungal potential against all selected microorganisms. The most sensitive bacterium in our study was Pseudomonas aeruginosa, while the tested defensive secretion manifested the lowest activity against Escherichia coli. The defensive secretion of A. insculpta also showed an ability, albeit mild, to suppress biofilm formation by P. aeruginosa. Among the tested yeasts, Candida albicans and C. krusei were the most susceptible and most resistant species, respectively. Finally, the concentration of extracts obtained from the tested defensive secretion needed to achieve an antifungal effect was lowest in the case of Cladosporium cladosporioides. Fusarium verticillioides and Penicillium rubens were the micromycetes most resistant to the tested secretion. Our results indicate that antibacterial activity of the defensive secretion of A. insculpta is similar to or slightly weaker than that of streptomycin, while comparison with antimycotics showed that the tested millipede secretion has stronger activity than fluconazole, but weaker activity than nystatin and ketoconazole. The present study corroborates previous findings indicating that the defensive secretions of millipedes can have different roles apart from antipredator protection and are effective against pathogenic microorganisms.

Multifaceted activity of millipede secretions: Antioxidant, antineurodegenerative, and anti-Fusarium effects of the defensive secretions of Pachyiulus hungaricus (Karsch, 1881) and Megaphyllum unilineatum (C. L. Koch, 1838) (Diplopoda: Julida)

Members of the millipede order Julida rely on dominantly quinonic defensive secretions with several minor, non-quinonic components. The free radical-scavenging activities of ethanol, methanol, hexane, and dichloromethane extracts of defensive secretions emitted by Pachyiulus hungaricus (Karsch, 1881) and Megaphyllum unilineatum (C. L. Koch, 1838) were investigated using the ABTS, DPPH, and total reducing power (TRP) tests. The obtained extracts were also tested for inhibition of acetylcholinesterase and tyrosinase activity. Finally, the antifungal potential of both julid extracts was evaluated against seven Fusarium species. Secretions of both species showed activity against free radicals, acetylcholinesterase, tyrosinase, and all of the selected fungal species. The secretions of P. hungaricus exhibited a more potent antioxidative effect than did those of M. unilineatum, while there were no significant differences of antiacetylcholinesterase activity between the tested extracts. Only the hexane extract of M. unilineatum showed an effect on tyrosinase activity stronger than that of P. hungaricus. Fusarium sporotrichioides, F. graminearum, and F. verticillioides were the fungi most resistant to secretions of both julids. The Fusarium species most susceptible to the secretion of P. hungaricus was F. avenaceum, while the concentrations of M. unilienatum extracts needed to inhibit and completely suppress fungal growth were lowest in the case of their action on F. lateritium. Our data support previous findings that julid defensive secretions possess an antimicrobial potential and reveal their antioxidative and antineurodegenrative properties. Bearing in mind the chemical complexity of the tested defensive secretions, we presume that they can also exhibit other biological activities.

Step-wise evolution of complex chemical defenses in millipedes: a phylogenomic approach

With fossil representatives from the Silurian capable of respiring atmospheric oxygen, millipedes are among the oldest terrestrial animals, and likely the first to acquire diverse and complex chemical defenses against predators. Exploring the origin of complex adaptive traits is critical for understanding the evolution of Earth's biological complexity, and chemical defense evolution serves as an ideal study system. The classic explanation for the evolution of complexity is by gradual increase from simple to complex, passing through intermediate "stepping stone" states. Here we present the first phylogenetic-based study of the evolution of complex chemical defenses in millipedes by generating the largest genomic-based phylogenetic dataset ever assembled for the group. Our phylogenomic results demonstrate that chemical complexity shows a clear pattern of escalation through time. New pathways are added in a stepwise pattern, leading to greater chemical complexity, independently in a number of derived lineages. This complexity gradually increased through time, leading to the advent of three distantly related chemically complex evolutionary lineages, each uniquely characteristic of each of the respective millipede groups.

A Model for Phylogenetic Chemosystematics: Evolutionary History of Quinones in the Scent Gland Secretions of Harvestmen

By the possession of unique exocrine scent glands, Opiliones (harvestmen) arise as a perfect model for studies on the evolutionary history of secretion chemistry. Among gland compounds of harvestmen, it is the quinones that represent recurring elements across the secretions of all suborders. Reliable data on quinone-distribution, however, is only known for Laniatores (benzoquinones) and Cyphophthalmi (naphthoquinones). We here unraveled the quinone-distribution across scent gland secretions of the third large harvestman suborder, the Palpatores (= Eu- and Dyspnoi): Naphthoquinones were found in phalangiid Eupnoi across all subfamilies as well as in nemastomatid (and at least one ischyropsalid) Dyspnoi. Benzoquinones (1,4-benzoquinone) were restricted to a small entity within Eupnoi, namely platybunine Phalangiidae, probably misplaced Gyantinae (currently Sclerosomatidae) and Amilenus (incertae sedis). Our findings, combined with data from Laniatores and Cyphophthalmi, allow evaluation of a comprehensive chemosystematic model for Opiliones for the first time. Evolutionary scenarios imply naphthoquinones as scent gland compounds of common ancestry, having evolved in an early harvestman ancestor and present in cyphophthalmids and palpatoreans, but lost in laniatoreans. Benzoquinones evolved later and independently at least twice: once in the secretions of gonyleptoid Laniatores (alkylated benzoquinones), and a second time in a lineage of phalangiid Eupnoi (1,4-benzoquinone).

Methyl N-methylanthranilate: major compound in the defensive secretion of Typhloiulus orpheus (Diplopoda, Julida)

The defensive secretion of the julid diplopod Typhloiulus orpheus contains methyl N-methylanthranilate (MNMA), an ester of N-methylanthranilic acid that comprises more than 99% of secretion of this species. MNMA is accompanied by small amounts of methyl anthranilate and two benzoquinones (2-methyl-1,4-benzoquinone and 2-ethyl-1,4-benzoquinone, respectively). MNMA is a known intermediate in the biosynthesis of both benzoquinones (as present in defensive secretions of juliformians) and glomerin-like quinazolines (chemical defense in Glomerida). The compound may have evolved independently in the pathway to glomeridan chemistry, or may even represent a pivotal branching point in the pathway to different chemical classes of diplopod defensive chemistry.

Chemical Ecology of Cave-Dwelling Millipedes: Defensive Secretions of the Typhloiulini (Diplopoda, Julida, Julidae)

Cave animals live under highly constant ecological conditions and in permanent darkness, and many evolutionary adaptations of cave-dwellers have been triggered by their specific environment. A similar "cave effect" leading to pronounced chemical interactions under such conditions may be assumed, but the chemoecology of troglobionts is mostly unknown. We investigated the defensive chemistry of a largely cave-dwelling julid group, the controversial tribe "Typhloiulini", and we included some cave-dwelling and some endogean representatives. While chemical defense in juliform diplopods is known to be highly uniform, and mainly based on methyl- and methoxy-substituted benzoquinones, the defensive secretions of typhloiulines contained ethyl-benzoquinones and related compounds. Interestingly, ethyl-benzoquinones were found in some, but not all cave-dwelling typhloiulines, and some non-cave dwellers also contained these compounds. On the other hand, ethyl-benzoquinones were not detected in troglobiont nor in endogean typhloiuline outgroups. In order to explain the taxonomic pattern of ethyl-benzoquinone occurrence, and to unravel whether a cave-effect triggered ethyl-benzoquinone evolution, we classed the "Typhloiulini" investigated here within a phylogenetic framework of julid taxa, and traced the evolutionary history of ethyl-benzoquinones in typhloiulines in relation to cave-dwelling. The results indicated a cave-independent evolution of ethyl-substituted benzoquinones, indicating the absence of a "cave effect" on the secretions of troglobiont Typhloiulini. Ethyl-benzoquinones probably evolved early in an epi- or endogean ancestor of a clade including several, but not all Typhloiulus (basically comprising a taxonomic entity known as "Typhloiulus sensu stricto") and Serboiulus. Ethyl-benzoquinones are proposed as novel and valuable chemical characters for julid systematics.

Chemical Defence in a Millipede: Evaluation and Characterization of Antimicrobial Activity of the Defensive Secretion from Pachyiulus hungaricus (Karsch, 1881) (Diplopoda, Julida, Julidae)

The chemical defence of the millipede Pachyiulus hungaricus is reported in the present paper, in which a chemical characterization is given and antimicrobial activity is determined. In total, independently of sex, 44 compounds were identified. All compounds belong to two groups: quinones and pentyl and hexyl esters of long-chain fatty acids. The relative abundances of quinones and non-quinones were 94.7% vs. 5.3% (males) and 87.3% vs. 12.7% (females), respectively. The two dominant quinones in both sexes were 2-methyl-1,4,-benzoquinone and 2-methoxy-3-methyl-1,4-benzoquinone. Antibacterial and antifungal activity of the defensive secretion was evaluated in vitro against seven bacterial strains and eight fungal species. With the aid of a dilution technique, the antimicrobial potential of the secretion and high sensitivity of all tested strains were confirmed. The lowest minimum concentrations of these compounds (0.20-0.25 mg/mL) were sufficient for inhibition of Aeromonas hydrophila, Listeria monocytogenes and Methicillin resistant Staphylococcus aureus (MRSA). The growth of eight tested fungal species was inhibited by slightly lower concentrations of the secretion, with Fusarium equiseti as the most sensitive fungus and Aspergillus flavus as the most resistant. Values of MIC and MFC in the employed microdilution assay ranged from 0.10 to above 0.35 mg/mL. The given extract contains antimicrobial components potentially useful as therapeutic agents in the pharmaceutical and agricultural industries.